U.S. patent number 5,126,300 [Application Number 07/719,987] was granted by the patent office on 1992-06-30 for clay composites for the removal of so.sub.x from flue gas streams.
This patent grant is currently assigned to Board of Trustees operating Michigan State University. Invention is credited to Jayantha Amarasekera, Thomas J. Pinnavaia, Christine A. Polansky.
United States Patent |
5,126,300 |
Pinnavaia , et al. |
June 30, 1992 |
Clay composites for the removal of SO.sub.x from flue gas
streams
Abstract
A method for preparing smectite clay alkaline earth metal
hydroxide and carbonate composite materials is described. The
method uses low amounts of the clay added to water to which is
added calcium oxide or calcium hydroxide. Optionally the calcium
carbonate is formed in situ by exposure to air or to carbon dioxide
in the solution. The product is dried to form the composite
material which is used to remove SO.sub.x from flue gases.
Inventors: |
Pinnavaia; Thomas J. (East
Lansing, MI), Polansky; Christine A. (Ithaca, MI),
Amarasekera; Jayantha (East Lansing, MI) |
Assignee: |
Board of Trustees operating
Michigan State University (East Lansing, MI)
|
Family
ID: |
24208742 |
Appl.
No.: |
07/719,987 |
Filed: |
June 24, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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553254 |
Jul 16, 1990 |
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Current U.S.
Class: |
502/84 |
Current CPC
Class: |
B01J
20/12 (20130101); B01J 37/031 (20130101); C01B
33/44 (20130101); B01J 37/0207 (20130101); B01D
53/508 (20130101); B01J 21/16 (20130101) |
Current International
Class: |
B01J
37/02 (20060101); B01J 37/03 (20060101); B01J
37/00 (20060101); C01B 33/00 (20060101); C01B
33/44 (20060101); B01J 20/10 (20060101); B01J
20/12 (20060101); B01J 21/00 (20060101); B01D
53/50 (20060101); B01J 21/16 (20060101); B01J
021/16 () |
Field of
Search: |
;502/84 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Josewicz et al., React. Solids, 6; 243 (1988). .
B. K. Gullett et al., React Solids, 3; 337 (1987). .
B. K. Gullett et al., React. Solids, 6; 263 (1988). .
Chang E. Y., et al., AIChE J., 30; 450 (1984). .
Thibault, J. D., et al., Can. J. Eng., 60; 796 (1982). .
Chang, J. C. S., et al., Envir. Prog., 3; 267 (1984). .
Fuller, E. L., et al., Langmuir, 3; 753 (1987). .
Jozewicz, W., et al., JAPCA, 38; 796 (1988). .
Jozewicz, W., et al. EPA/600; D-87/095, (NTIS PB87-175857/AS).
.
Jozewicz, W., et al., EPA/600/D-87/135, (NTIS PB87-182663). .
Chang et al., "Fossil Fuels Utilization:Environmental Concerns"
Eds. R. Markuszewski, B. Blaustein, Chap. 15. .
"Crystal Structures of Clay Minerals and Their X-ray
Identification" Eds. Brindley et al., Chap. 1 and 3. .
Laszlo, P., Science, 235; 1473 (1987)..
|
Primary Examiner: Dees; Carl F.
Attorney, Agent or Firm: McLeod; Ian C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. application Ser.
No. 07/553,254, filed Jul. 16, 1990.
Claims
We claim:
1. A method for preparing a composite material useful for SO.sub.x
removal from the flue gas of a coal-burning power plant and other
gas streams which comprises:
(a) providing a suspension of a smectite clay containing 0.1 to 5%
by weight of clay in water;
(b) adding a basic material selected from the group consisting of
alkaline earth metal oxides and hydroxides to the clay suspension
to provide a resulting suspension;
(c) mixing the resulting suspension;
(d) recovering a precipitate from the resulting suspension; and
(e) drying the precipitate at a temperature between
100.degree.-120.degree. C. to provide the composite material,
wherein when the composite material is heated to a temperature of
500.degree. C. or above, the SO.sub.x components from a gas stream
are removed by the composite material upon contacting with the said
gas stream at that temperature.
2. The method of claim 1 wherein the weight ratio of the basic
material to clay in the composite material is between 1:1 and
10:1.
3. A method in accordance with claim 1 wherein the composite
material is dried as a cake, crushed, and then sieved to separate
desired mesh size ranges.
4. A method in accordance with claim 1 wherein the smectite clay is
selected from the group consisting of montmorillonite,
fluorohectorite, bentonite, nontronite, hectorite, saponite, and
beidellite, preferably in Na.sup.+ exchanged form.
5. A method in accordance with claim 1 wherein the alkaline earth
metal is selected from the group consisting of magnesium and
calcium.
6. A method in accordance with claim 1 wherein the composite
material is substantially in Ca(OH).sub.2 with minor amounts of
CaCO.sub.3 as the basic material.
7. A method for preparing a composite material useful for SO.sub.x
removal from the flue gas of a coal-burning power plant and other
gas streams which comprises:
(a) providing a suspension of a smectite clay containing 0.1 to 5%
by weight of clay in water;
(b) adding a basic material selected from the group consisting of
alkaline earth metal oxides and hydroxides to the clay suspension
to provide a resulting suspension;
(c) mixing the resulting suspension by stirring with CO.sub.2 gas
present in water to convert at least some of the basic material to
alkaline metal carbonate;
(d) recovering a precipitate from the resulting suspension; and
(e) drying the precipitate in air at ambient temperatures to
provide the composite material, wherein when the composite material
is heated to a temperature of 500.degree. C. or above, the SO.sub.x
components from a gas stream are removed by the composite material
upon contacting with the said gas stream at that temperature.
8. The method of claim 7 wherein basic weight ratio of the basic
material to clay in the composite material is between 1:1 and
10:1.
9. A method in accordance with claim 7 wherein the composite
mateiral is dried in a cake, crushed, and then sieved to separate
desired mesh size ranges.
10. A method in accordance with claim 7 wherein the smectite clay
is selected from the group consisting of montmorillonite,
fluorohectorite, bentonite, nontronite, hectorite, saponite, and
beidellite, preferably Na.sup.+ exchange form.
11. A method in accordance with claim 7 wherein the alkaline earth
metal is selected from the group consisting of magnesium and
calcium.
12. A method in accordance with claim 7 wherein the composite
material is substantially CaCO.sub.3 with minor amounts of
Ca(OH).sub.2 as the basic material.
13. A method for preparing a composite material useful for SO.sub.x
removal from the flue gas of a coal-burning power plant and other
gas streams which comprises:
(a) providing a suspension of a smectite clay containing 0.1 to 5%
by weight of clay in water;
(b) adding a basic material seelcted from the group consisting of
alkaline earth metal oxides and hydroxides to the clay suspension
as a resulting suspension;
(c) mixing the resulting suspension by stirring while bubbling
CO.sub.2 gas through the suspension to convert at least some of the
basic material to alkaline metal carbonate in the resulting
suspension;
(d) recovering a precipitate from the water by filtration or by
centrifugation; and
(e) drying the precipitate in air at ambient temperatures or in an
oven at 110.degree. C. to prvoide the composite material, wherein
when the composite material is heated to a temperature of
500.degree. C. or above, the SO.sub.x components from a gas stream
are removed by the composite material upon contacting with the said
gas stream at that temperature.
14. The method of claim 13 wherein the basic compound weight ratio
of basic material to clay in the composite is between 1:1 and
10:1.
15. A method in accordance with claim 13 wherein the composite
material is dried in a cake, crushed, and then sieved to separate
the desired mesh sizes.
16. A method in accordance with claim 13 wherein the smectite clay
is selected from the group consisting of montmorillonite,
fluorohectorite, bentonite, nontronite, hectorite, saponite, and
beidellite, preferably in Na.sup.+ exchange form.
17. A method in accordance with claim 13 wherein the alkaline earth
metal is selected from the group consisting of magnesium and
calcium.
18. A method in accordance with claim 13 wherein the composite is
substantially in calcium carbonate with minor amounts of calcium
hydroxide as the basic material.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to the use of smectite clay composites
containing alkaline earth metal hydroxides and carbonates for the
removal of SO.sub.x, (sulfur dioxide and sulfur trioxide), from
flue gases, particularly flue gas from coal burning power plants,
and to the method of preparing them.
(2) Prior Art
The first example of flue gas scrubbing for sulfur dioxide control
occurred in London, England, in 1933. However, the application of
this technology to coal-fired utility boilers in the United States
did not begin until the 1970's. The first large-scale application
of flue gas scrubbing using lime or limestone was installed in
1964, in the Soviet Union. This facility was followed by an
installation at a large sulfuric acid plant in Japan in 1966. In
1970, the Clean Air Act Amendments were adopted. This legislation
provided for enforcement, by the United States Environmental
Protection Agency (EPA), of SO.sub.x emissions limits for power
plants constructed or modified after Aug. 17, 1971. This Act
spurred extensive flue gas desulfurization (FGD) research. As of
Jan. 1984, calcium based, wet, throwaway systems (including lime,
limestone, and alkaline-ash systems) accounted for 84 percent of
existing and planned FGD capacity. The Clean Air Act was amended in
1977 to require further control of SO.sub.x emissions. Increasing
federal regulations and the high cost to construct and operate
existing wet FGD units have encouraged continued research on new or
modified flue gas cleanup processes.
Controlling the emissions of SO.sub.x from power plants is a
world-wide problem due to its relationship to "acid rain".
Therefore, research into its control is a global effort. Example of
a recent patent using calcium based systems to reduce SO.sub.x
emissions is Thompson and Nuzio, U.S. Pat. No. 4,731,233. In most
cases a commercial source of limestone or lime is used due to cost
effectiveness and available quantities.
There are a number of ways to control SO.sub.x emissions in
existing power plants or features that can be included in
construction of new power plants. These approaches can be
classified according to the position in the combustion system at
which pollutant control technology is applied. Precombustion
control involves removal of sulfur, nitrogen and ash compounds from
the fuel before it is burned. In most cases this involves
application of coal-cleaning technology. Combustion control
includes staged combustion, boiler limestone injection, and
fluidized-bed combustion with limestone addition. Post-combustion
control involves removal of pollutants after they have been formed
but before they are released into the atmosphere. This would
include in-duct dry sorbent injection, induct spray drying and
combined electrostatic precipitator (ESP)/fabric filter sorbent
injection (Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna,
T. G., React. Solids, 6 243 (1988)).
U.S. Pat. No. 4,981,825 to Pinnavaia and Moini, describes the use
of smectite clays mixed with metal oxide sol particles to prevent
sintering of the clays when heated to elevated temperatures. This
method is more complicated than necessary with calcium hydroxide,
calcium oxide or other metal oxides which are reactive with
SO.sub.x.
Flue gas treatment systems can be classified as either wet or dry
based on the moisture content of the treated flue gas and the spent
sorbent. Wet systems completely saturate the flue gas with water
vapor. The flue gas is contacted with a liquid or slurry stream.
Dry systems contact the flue gas with a dry or wet sorbent but
never include enough water for complete saturation of the flue gas.
Dry systems result in a dry product or spent sorbent material,
while wet systems results in either a slurry or a sludge.
Although calcium based systems are the major source of SO.sub.x
control, they are not without problems. Agglomeration of particles
can be a serious problem that results in a less than optimal
conversion to CaSO.sub.x, (CaSO.sub.3 and CaSO.sub.4). The activity
of the calcium species decreases as its particle size increases.
Also CaSO.sub.x occupies more volume than CaO, the common active
species. Therefore, an increase in volume occurs as the reaction
proceeds, which causes a loss in the original porous character of
the CaO. This results in a blockage of SO.sub.x and O.sub.2 to the
active CaO centers (Gullett, B. K. and Blom, J. A., React Solids, 3
337 (1987); Gullett, B. K., Blom, J. A. and Cunningham, R. T.,
React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J.,
30 450 (1984); Thibault, J. D., Steward, F. R. and Ruthven, D. M.,
Can. J. Chem. Eng., 60 796 (1982)).
Prior Art in this field has used limestone, lime or hydrated lime
as a precursor for the active CaO species or has used Ca(OH).sub.2
as the active species. Generally, the active species has been used
as a bulk phase and not as a dispersed species (Chang, J. C. S. and
Kaplan, N., Envir. Prog., 3 267 (1984); Gullett, B. K., Blom, J. A.
and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y.
and Thodes, G., AIChE J., 30 450 (1984); Fuller, E. L. and Yoos, T.
R., Langmuir, 3 753 (1987)). Recent work has concentrated on the
addition of fly ash to Ca(OH).sub.2 to enhance its activity in
SO.sub.x control (Jozewicz, W. and Rochelle, G. T., Envir. Prog, 5
219 (1986); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna,
T. G., JAPCA, 38 796 (1988); Jozewicz, W., Chang, J. C. S., Sedman,
C. B. and Brna, T. G., React. Solids, 6 243 (1988); Jozewicz, W.,
Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600;D-87/095,
(NTIS PB87-175857/AS); Jozewicz, W., Chang, J. C. S., Sedman, C. B.
and Brna, T. G., EPA/600/D-87/135, (NTIS PB87-182663). The fly ash
is a siliceous material and formation of various calcium silicates
can occur. Several diatomaceous earths, montmorillonite clays and
kaolins have also been identified as containing reactive silica
(Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G.,
React. Solids, 6 243 (1988)).
Conventional systems use limestone, lime, or hydrated lime as a
precursor for the reactive CaO species or have used Ca(OH).sub.2 as
the reactive species. Agglomeration of particles is a serious
problem that results in less than optimal conversion to CaSO.sub.x
for all of these systems. The activity of the calcium species
decreases as its particle size increases. This is caused by the
larger volume that CaSO.sub.x occupies compared to CaO, the common
active species. An increase in volume occurs as the reaction
proceeds, which causes a loss in the original porous character of
the CaO. This results in a blockage of SO.sub.x and O.sub.2
resulting in inefficient removal of SO.sub.x from flue gas streams
(Gullett, B. K. and Blom, J. A., React. Solids, 3 337 (1987);
Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6
263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984);
Thibault, J. D., Steward, F. R. and Ruthven, D. M., Can. J. Chem.
Eng., 60 796 (1982)).
OBJECTS
Therefore, an object of the present invention is to provide an
improved synthesis of alkaline earth metal carbonate or alkaline
earth metal hydroxide containing clay composites which are suitable
for removing SO.sub.x components from flue gas streams. In the
preferred improved process, lime is used as the precursor base
component along with smectite type clays, which is then converted
into slaked lime (Ca(OH).sub.2) in water or to CaCO.sub.3 in the
presence of carbon dioxide, rather than by introducing the
CaCO.sub.3 directly into the clays.
Another object of the present invention is to provide highly
reactive basic sorbent compositions for the removal of SO.sub.x
components from flue gas streams, especially from coal burning
power plants. The smectite type clays serves as a support for the
reactive base, as a dispersing agent for improved reactivities of
the base components towards SO.sub.x, and when structural iron ions
are present, as a catalyst for oxidation of SO.sub.x to SO.sub.3,
which is more reactive toward the supported base.
GENERAL DESCRIPTION
The present invention relates to a method for preparing a composite
material useful for SO.sub.x removal from the flue gas of a
coal-burning power plant and other gas streams which comprises:
providing a suspension of a smectite clay containing 0.1 to 5% by
weight of clay in water; adding a basic material selected from the
group consisting of alkaline earth metal oxides and hydroxides to
the clay suspension to provide a resulting suspension; mixing the
resulting suspension; recovering a precipitate from the resulting
suspension; and drying the precipitate at a temperature between
100.degree.-120.degree. C. to provide the composite material,
wherein when the composite material is heated to a temperature of
500.degree. C. or above, the SO.sub.x components from a gas stream
are removed by the composite material upon contacting with the said
gas stream at that temperature.
The present invention also relates to a method for preparing a
composite material useful for SO.sub.x removal from the flue gas of
a coal-burning power plant and other gas steams which comprises:
providing a suspension of a smectite clay containing 0.1 to 5% by
weight of clay in water; adding a basic material selected from the
group consisting of alkaline earth metal oxides and hydroxides to
the clay suspension to provide a resulting suspension; mixing the
resulting suspension by stirring with CO.sub.2 gas present in water
to convert at least some of the basic material to alkali metal
carbonate; recovering a precipitate from the resulting suspension;
and drying the precipitate in air at ambient temperatures to
provide the composite material, wherein when the composite material
is heated to a temperature of 500.degree. C. or above, the SO.sub.x
components from a gas stream are removed by the composite material
upon contacting with the said gas stream at that temperature.
The present invention further relates to a method for preparing a
composite material useful for SO.sub.x removal from the flue gas of
a coal-burning power plant and other gas streams which comprises:
providing a suspension of a smectite clay containing 0.1 to 5% by
weight of clay in water; adding a basic material selected from the
group consisting of alkaline earth metal oxides and hydroxides to
the clay suspension as a resulting suspension; mixing the resulting
suspension by stirring while bubbling CO.sub.2 gas through the
suspension to convert at least some of the basic material to alkali
metal carbonate in the resulting suspension; recovering a
precipitate from the water by filtration or by centrifugation; and
drying the precipitate in air at ambient temperatures or in an oven
at 110.degree. C. to provide the composite material, wherein when
the composite material is heated to a temperature of 500.degree. C.
or above, the SO.sub.x components from a gas stream are removed by
the composite material upon contacting with the said gas stream at
that temperature.
Further, the present invention relates to a method for removing
SO.sub.x from a gas mixture which comprises: providing a dry
physical mixture of a basic material selected from the group
consisting of an alkali metal oxide and an alkali metal hydroxide
and a smectite clay; mixing the dry physical mixture to insure
homogeneity of the mixture; adding a minimum amount of water to the
dry physical mixture to make a paste; blending the paste in a
carbon dioxide atmosphere to promote the formation of an alkali
metal carbonate from the basic material; drying the paste at a
temperature up to about 110.degree. C. to provide the composite
material, wherein when the composite material is heated to a
temperature of 500.degree. C. or above, the SO.sub.x components
from a gas stream are removed by the composite material upon
contacting with the said gas stream at that temperature.
Our application Ser. No. 07/553,254 describes clay-composite
materials useful for removing SO.sub.x from a gas stream. In this
patent application, alkali and alkaline earth oxides or carbonates
such as NaHCO.sub.3 and CaCO.sub.3 were precipitated in the
presence of smectite clay suspensions from respective salts or was
impregnated onto clay using water soluble compounds. When these
composite materials are heated to at least 500.degree. C. the
SO.sub.x is removed from the gas streams by the basic compound.
In accordance with one method of the previous invention, a 0.5 to
1.5 weight percent, wt. %, aqueous suspension of clay was initially
prepared. An aqueous solution of Na.sub.2 CO.sub.3 was added
dropwise to the clay suspension while it was stirred. This was
followed by a similar addition of CaCl.sub.2 .multidot.2H.sub.2 O.
The addition of the calcium species caused the precipitation of
CaCO.sub.3. The amount of Na.sub.2 CO.sub.3 and CaCl.sub.2
.multidot.2H.sub.2 O was varied to provide the desired weight ratio
of CaCO.sub.3 to clay. The product was washed with deionized
distilled water, either by centrifugation/decantation or by
dialysis, to remove the excess chloride and sodium ions before
drying. Washing the preparation was preferred, because reactivity
with SO.sub.x was diminished if no attempt was made to remove the
chloride. The adverse effect of chloride on SO.sub.x removal has
also been verified by another study which evaluated the effects of
magnesium and chloride ions on the performance of
limestone-regenerated dual alkali processes under closed-loop
operating conditions (Chang, J. C. S., Kaplan, N. and Brna, T. G.
in "Fossil Fuels Utilization: Environmental Concerns" (Eds. R.
Markuszewski, B. Blaustein) Chap. 15). Limestone reactivity
decreased with the increase of chloride ion concentration. The
effect was especially pronounced after a concentration of 80,000
ppm was reached. The base-clay composites were thus, thoroughly
washed by employing several wash cycles.
One difficulty with this earlier process of making base-clay
composites is this extensive washing procedure involved during the
synthesis to remove chloride ions. In an industrial scale
preparation, this would not be economically feasible.
The composites of the present invention are base-smectite clay
composites. The present invention provides a method wherein
composite materials of highly dispersed bases are provided on clay
supports. This results in decrease of agglomeration of base
particles (e.g. CaO, Ca(OH).sub.2 or CaCO.sub.3), and in an
increase in the optimal conversion to CaSO.sub.x that can be
achieved.
The common bases employed are CaO, Ca(OH).sub.2 and CaCO.sub.3 but
are not exclusive to these. Other base materials are magnesium
oxide, magnesium carbonate, magnesium hydroxide, zinc oxide, zinc
carbonate, zinc hydroxide, aluminum oxide, and aluminum hydroxide.
The base can be derived from any alkaline earth metal salt such as
magnesium or from any alkali metal salt, including those of sodium,
lithium, potassium, and the like.
The clays utilized in this invention are members of the smectite
group of hydrous 2:1 layer lattice silicates containing
two-dimensional tetrahedral sheets of composition T.sub.2 O.sub.5
(T=tetrahedral cation, i.e. Si.sup.4+, Al.sup.3+, or Fe.sup.3+), in
which individual tetrahedra are linked with neighboring tetrahedra
by sharing three corners each (the basal oxygens) to form an
hexagonal mesh pattern. The fourth tetrahedral corner (the apical
oxygen) points in a direction normal to the sheet and at the same
time forms part of the immediately adjacent octahedral sheet in
which individual octahedra are linked laterally by sharing
octahedral edges. The octahedral cations are usually Mg.sup.2+,
Al.sup.3+, Fe.sup.2+, and Fe.sup.3+, but other medium-sized cations
also occur in some species. The presence of iron in the clay can be
beneficial at SO.sub.2 uptake temperatures of 700.degree. C. or
less, since iron centers catalyze the oxidation of SO.sub.2 to more
reactive SO.sub.3 in this temperature range.
The smallest structural unit of a smectite clay contains three
octahedra. If all three octahedra are occupied, the sheet is
classified as trioctahedral. If only two octahedra are occupied and
the third octahedron is vacant, the sheet is classified as
dioctahedral. The assemblage formed by linking two-tetrahedral
sheets with one octahedral sheet is known as a 2:1 layer. The
linkage is achieved by inverting the upper tetrahedral sheet so
that its apical oxygens point down and can be shared by the
octahedral sheet below. Both octahedral anion planes then are of
the same O, OH composition. If the 2:1 layers are not
electrostatically neutral, the excess layer charge is neutralized
by various interlayer materials, including individual cations, and
hydroxide octahedral groups and sheets ("Crystal Structures of Clay
Minerals and Their X-ray Identification" (Eds. Brindley, G. W. and
Brown, G.) Chap. 1.)
Smectites are a type of 2:1 layered silicates. General formulas for
di- and trioctahedral smectites are as follows:
per unit cell. These 2:1 layered silicates have an electron
charge/unit cell between 0.4 and 1.2. Montmorillonite is the most
common smectite and refers to the dioctahedral Al, Mg smectite with
the layer charge arising mainly from the Mg-for-Al substitutions in
the octahedral position, i.e. (M.sub.y.sup.+ nH.sub.2 O)(Al.sub.4-y
Mg.sub.y)Si.sub.8 O.sub.20 (OH).sub.4 per unit cell.
Montmorillonites generally have total specific surface areas of the
order of 500-850 m.sup.2 /g, which includes both the interlayer and
eternal surface area ("Crystal structures of Clay Minerals and
Their X-ray Identification" (Eds. Brindley, G. W. and Brown, G.)
Chap. 8, and Laszlo, P. Science, 235 1473 (1987)).
Smectite clays also have the ability to swell in water. The
swelling ability of the clay depends upon whether a monovalent or
divalent cation is used to neutralize the clay layers. Monovalent
ions tend to remain more or less associated with the silicate
layers when hydration occurs but divalent ions tend to move into
the water layers. Thus, the silicate layers and their associated
monovalent ions behave in a similar manner to neutral entities with
the layers becoming fully dispersed in water. Divalent ion clays
appear to be dispersesd in small packets generally compromising 4-5
layers. Divalent ions link pairs of layers together by satisfying
two negative charges in a manner which monovalent ions cannot.
The formation of a highly dispersed clay suspension facilitates a
uniform dispersion of the base particles during preparation of the
base/clay composite. The dispersed base of the composite material
possesses a different dispersion than the bulk base. The greater
dispersion of the base particles in the composite is conducive to
the reaction of the base with SO.sub.x resulting in a more
efficient use of the available base. An improvement in SO.sub.x
reactivity has been observed with composites containing only small
amounts of clay. A ratio of 9:1 base to clay still results in
improved uptake over conventional bulk base systems.
The present invention provides methods for the production of
composite materials consisting of alkaline earth metal bases and
smectite clay composites of varying alkaline earth metal base to
clay ratios than has heretofore been known in the prior art,
especially, and most preferably, by causing the formation of
Ca(OH).sub.2 or CaCO.sub.3 from CaO onto and between clay particles
while the dispersed clay is in aqueous suspension. The resulting
composite materials are suitable for removing SO.sub.x from the
flue gas of coal burning power plants and other gas streams.
SPECIFIC DESCRIPTION
In the first method disclosed in this invention, a suspension of
lime (CaO) in water is added to a stirred suspension of smectite
clay in water. Alternatively, CaO can be added as a solid into the
stirred clay slurry. The action of water on CaO results in the
formation of Ca(OH).sub.2 in essentially quantitative yields, as
judged by x-ray powder diffraction. The stirring procedure is
carried out to facilitate the proper dispersion of basic
Ca(OH).sub.2 particles onto the smectite clay platelets. The
composite thus obtained is recovered either by filtration or by
centrifugation and dried in air or in an oven at 110.degree. C. to
isolate the dry composite material.
Two preferred forms of smectite clays have been employed in this
invention, namely, Na-montmorillonite from Crook County, Wyo., USA
and Ca-montmorillonite from Apache County, Ariz., USA. The type of
smectite clay is, however, not limited to montmorillonites. Other
types of smectites such as hectorite, fluorohectorite, saponite,
bentonite, beidellite, nontronite, and the like also serve as good
supports to dispersed base particles in the composites.
It is important that clay suspension contains less than 5% w/w
clay. At higher concentrations these slurries tend to form gels and
thus making the subsequent mixing process with CaO base difficult.
It is more preferred that the clay slurries contain between 1-2%
w/w clay. At these lower concentrations the dispersion of base
particles within clay is much more efficient and results in
formation of composites with very highly dispersed base
particles.
The amount of CaO used depends on the sorbent/clay ratio desired.
Different ratios of CaO to clay were tested. Particularly good
SO.sub.x reactivities were observed when calcium containing base to
clay ratio is between 1:1 to 10:1.
Instead of CaO as the base precursor, one may use Ca(OH).sub.2 or
CaCO.sub.3 in preparing these clay-base composites. Accordingly, in
another preferred method of the present invention, the preparation
of Ca(OH).sub.2 -clay composite is accomplished using Ca(OH).sub.2
from Columbus, Ohio. The base-clay composite isolated showed
reactivities towards SO.sub.x comparable to the composites prepared
using CaO as a precursor.
The reaction of CaO/clay mixture with water converts CaO to
Ca(OH).sub.2 during the preparation of the above the
clay-containing composites. X-ray powder diffraction (XRPD) data on
these solid composites indicate that some or all of the
Ca(OH).sub.2 thus formed can be converted to calcium carbonate when
the composite is exposed to carbon dioxide. Composites prepared
directly from Ca(OH).sub.2 and clay suspensions also behave in this
manner when exposed to CO.sub.2. Even the CO.sub.2 in the ambient
atmosphere is sufficient to convert some Ca(OH).sub.2 to
CaCO.sub.3. However, by minimizing the exposure of the wet
composites to CO.sub.2 by drying quickly in an oven at 110.degree.
C., one can largely avoid the CaCO.sub.3 formation. The composites
isolated in this manner were rich in Ca(OH).sub.2. On the other
hand, by exposing the wet Ca(OH).sub.2 /clay composites to an
atmosphere rich in CO.sub.2, one can prepare a clay composite
containing calcium largely in the form of CaCO.sub.3. The
composites which are rich in CaCO.sub.3 showed better reactivities
towards SO.sub.x.
The present invention particularly relates to a method for
preparing these clay composite materials that are rich in
CaCO.sub.3, which are suitable for removing SO.sub.x from a flue
gas and other gas streams, by adding CaO or Ca(OH).sub.2 as solids
or as suspension in water to a smectite clay suspension in water.
The resulting slurry is treated with a stream of CO.sub.2 gas. XRPD
studies of the products isolated showed that the conversion of CaO
or Ca(OH).sub.2 to CaCO.sub.3 was completed after about 30 minutes
to 1 hour, depending on the amounts of materials being treated. The
products are isolated as before and either dried in air or in an
oven at 110.degree. C. Instead of purging CO.sub.2 gas through
these slurries, one may stir these slurries in air for longer
periods of time to provide ample time to convert Ca(OH).sub.2 in
the slurries to CaCO.sub.3 from atmospheric CO.sub.2.
Another preferred method of the present invention provides a
composite material for SO.sub.x removal from flue gas by starting
with a dry physical mixture of CaO or Ca(OH).sub.2 and clay. The
dry physical mixture is prepared by any suitable blending procedure
such as ball milling, grinding, and the like. A paste then is
prepared from the blended powder by using the minimum amount of
water to thoroughly wet the sample. Carbon dioxide gas, CO.sub.2,
is blended into the paste to form the CaCO.sub.3 /clay composite.
The paste is dried in air at 25.degree. to 100.degree. C.
Composites prepared by this method are superior to dry physical
mixtures of CaO or Ca(OH).sub.2 and clay towards the reactivity
with SO.sub.2. Also, the composites prepared in this manner,
require minimum amounts of water. Thus, the paste formation is a
convenient route to producing CaCO.sub.3 -containing clay
composites, especially on an industrial scale. The paste formed can
be extruded easily to any shape and size before drying. The dry
composites isolated can be introduced at different stages in the
coal-burning power plants.
These methods developed in this invention result in the formation
of Ca(OH).sub.2 and/or CaCO.sub.3 clay composites without the use
of a soluble base or a soluble base precursor. Further, the methods
disclosed here do not require time consuming steps of washing the
(Ca(OH).sub.2 and/or CaCO.sub.3)/clay composites to remove excess
sodium and chloride ions. Therefore, the methods described in this
patent are the preferred ones for the formation of Ca(OH).sub.2
/clay composites, CaCO.sub.3 /clay composites, or mixed
Ca(OH).sub.2 and CaCO.sub.3 /clay composites of varying ratios,
which are used to remove SO.sub.x from flue gas of coal-burning
power plants
In preparing these base/clay composites, lime is the preferred base
precursor, since it can be easily converted to highly reactive
Ca(OH).sub.2 in the presence of aqueous clay slurries or to
CaCO.sub.3 by exposing to CO.sub.2 gas. The addition of carbon
dioxide has the added advantage of further reducing the particle
size of these composites and enhancing the SO.sub.x reactivities.
Moreover, base precursors, such as CaCO.sub.3 and Ca(OH).sub.2 can
be obtained starting with soluble calcium salts such as calcium
chloride, calcium nitrate, calcium acetate, calcium oxalate, etc.,
and a suitable water soluble hydroxides such as sodium hydroxide,
ammonium hydroxide or a carbonate such as sodium carbonate, etc.
Furthermore, the base used in this invention is not limited to
calcium salts.
The composites disclosed in this invention show very good
rectivities towards SO.sub.2 at temperatures above 500.degree. C.
Thus, these composites can be pre-calcined at or above 500.degree.
C. before introducing to the coal-burning power plants. Drying and
calcination can take place simultaneously when the sorbents are
directly injected into the combustor.
There are several advantages of using smectite clays in these
clay/base composites. The presence of highly swellable smectite
clay allow the base particles to disperse on clay particles in
water, thus helping to minimize aggregation and sintering of the
base particles. As a result, the composites show high SO.sub.x
reactivities. The methods disclosed here for the formation of
base/clay composites provide materials superior to those prepared
by a dry physical mixture of CaO or Ca(OH).sub.2 and clay. For
instance, a fifty percent increase in SO.sub.2 reactivity was
observed when the wet method of preparation from Ca(OH).sub.2 was
used to form the CaCO.sub.3 /clay composite. Even greater increase
in SO.sub.2 reactivity were observed when the wet method of
preparation from CaO was used to form the CaCO.sub.3 /clay
composites.
Furthermore, the presence of clay make these composite particles
rigid and less fragile than the particles of base in the absence of
the clay component. This allows easy processing of the composite to
any form of particles differing sizes or shapes. The Ca(OH).sub.2
formed from the slaking of lime and the CaCO.sub.3 formed from the
carbonation of lime or slaked lime exist as fine particles and show
enhanced SO.sub.x reactivities than unprocessed lime. In practice,
however, it is difficult to collect these products via normal
filtration processes due to their fine particulate nature. In the
presence of smectite clays, as described in this invention, the
hydration and carbonation processes in the presence of smectite
clay give composites that are easily filterable. In other words,
clay greatly facilitates the filtering process, particularly when
the particles of Ca(OH).sub.2 and CaCO.sub.3 are small and
especially reactive towards SO.sub.x.
The composites prepared according to the present invention have a
ceramic texture suitable for withstanding attrition. Thus, in a
coal-fired boiler application, the present sorbents may be injected
to the combustion zone, (e.g., the boiler, temp.
700.degree.-1000.degree. C.) when combustion takes place, or added
with coal. Sorbents then leave the combustion zone with coal ash
and can be removed from the bag house. This process in turn,
provides enough contact time for the sorbents to react with
SO.sub.x from the flue gas streams. Thus the flue gas leaving the
combustion zone/contacting zone systems have reduced amounts of
sulfur oxide relative to the processing in the absence of present
sorbents. Due to the presence of the clay support, the reacted
sorbents also have the ceramic texture, which is ideal for the safe
deposition without any serious environmental pollution.
In a broader sense this invention considers the use of these clay
composites in controlling the sulfur oxides from gas streams, more
particularly from coal-fired boiler systems. These systems include
a boiler, economizer and dust collectors such as electrostatic
precipitator or bag filter house ("bag house"). The injection of
the sorbents into these, particularly to the boiler
(700.degree.-1000.degree. C.), along with coal has been considered
in this invention.
The clay composites prepared as described above, were thermally
treated in a temperature programmed thermogravimetric balance at a
temperature range of 500.degree.-1000.degree. C. in a stream of
air, and SO.sub.2 gas was introduced. The amount of SO.sub.2
reacted with the sorbents was monitored as the weight uptake.
Heating the composites to 900.degree. C. converts the Ca(OH).sub.2
or CaCO.sub.3 component to form CaO. Exposing the heated composite
to SO.sub.x in a gas stream containing some oxygen converts the CaO
rapidly to CaSO.sub.4. However, in a particular application it is
not necessary to pre-calcine the composite before exposing the
composite to the SO.sub.x stream.
The reactivities of representative clay-base composites with
SO.sub.2 at 900.degree. C. are given in the following Examples and
Table 1 following the Examples. All these composites show higher
reactivities with SO.sub.2 that are substantially greater than pure
CaO, or CaO derived from the thermal decomposition of Ca(OH).sub.2
or CaCO.sub.3. For example, the CaCO.sub.3 /Ca.sup.2+
-montmorillonite composite formed by reacting a 5:1,
CaO:Ca-montmorillonite with CO.sub.2 (8.9:1 as CaCO.sub.3
:Ca-montmorillonite) resulted in a conversion of 74.2% of Ca sites
in the composite to CaSO.sub.4 when the material is exposed 0.5%
v/v SO.sub.2 in a stream of air (200 ml/min) for one hour. During
the first 5 minute period 58.9% of the Ca sites were converted to
CaSO.sub.4. The excellent reactivity makes this composite an
attractive one for the removal of SO.sub.x from coal-burning power
plants. Comparable reactivities were also observed for other clay
composites containing different CaCO.sub.3 /clay ratios (Table 1).
A series of related composites were prepared by changing the
precursor base from CaO to Ca(OH).sub.2 or by changing the type of
clay from Ca-montmorillonite to Na-montmorillonite. All these
composites show very good reactivities towards SO.sub.x (Table
1).
EXAMPLES 1 to 3
The preparation of CaCO.sub.3 /Ca-montmorillonite composites of
different CaCO.sub.3 :clay ratios are described in this example.
The carbon dioxide gas from air or from the water present in the
reaction mixture is allowed to react with base component to form
CaCO.sub.3.
Ca-montmorillonite (Cheto) from Apache County, Ariz., USA was
selected as the representative member of the smectite family of 2:1
layer lattice silicates. A 1.8 weight percent, wt. %, of clay was
dispersed in deionized distilled water. An upper limit of 2 micron
particle size was achieved by sedimentation in water and
application of Stokes law of settling under gravity. The procedure
was performed twice to optimize clay purity. Sedimentation also
removed quartz and other insoluble inpurities that may have been
present in the clay. After purification, the clay was air dried on
a glass plate or stored in an aqueous suspension. In a large scale
application, it would not be necessary to purify the clay starting
material, instead, the clay ore could be used directly.
The preparation of CaCO.sub.3 :Ca-montmorillonite 5.4:1 (w/w) ratio
is described below.
The desired weight ratio, 5.4:1 of precipitated CaCO.sub.3
:Ca-montmorillonite was achieved by using 3 parts of lime, CaO to
one part of clay. A 0.75 g of pulverized CaO from Mississippi Lime
Company, Ste. Genevieve, Mo. was added slowly to a 100 ml of
deionized distilled water while stirring in the open atmosphere for
reaction with ambient CO.sub.2. A 100-ml portion of 0.25 wt. %
suspension of Ca-montmorillonite in deionized distilled water was
added to the first solution while stirring. The precipitate was air
dried at ambient temperature. An XRPD pattern of the product showed
a reflection at 5.8.degree. (15.2 .ANG.) characteristic of
Ca-montmorillonite peak and a peak at 29.5.degree. (3.0 .ANG.) due
to precipitated CaCO.sub.3.
A sample prepared as described above was evaluated and was shown to
be active for SO.sub.2 removal from a gas mixture. The sample was
heated at 5.degree. C./minutes to 900.degree. C. and held at
900.degree. C. for 30 minutes prior to the introduction of 5000 ppm
SO.sub.2 at 900.degree. C. for 1 hour in flowing air. The
CaCO.sub.3 decomposes to CaO at 900.degree. C. The conversion was
based upon the following reaction:
CaO+SO.sub.2 +O.sub.2 .fwdarw.CaSO.sub.4 was 74.2% after 1 hour of
reaction with 58.9% occurring within the first 5 minutes of
reaction (entry S.#1, in Table 1).
A similar procedure was used to produce a 8.9:1 (w/w) CaCO.sub.3
:Ca-montmorillonite composite (or 5:1 CaO:Ca-montmorillonite)
achieved by preparing a 100 ml of 0.2 wt. % suspension of clay in
deionized distilled water, adding a 1.0 g quantity of CaO (from the
source in Example 1) slowly to the clay suspension while stirring
in the open atmosphere, and drying the product in air at ambient
temperature. The product, designated sample 2 (S#2, Table 1), was
tested for SO.sub.2 reactivity at 900.degree. C. in flowing air
under the conditions cited above. After 1 hour of reaction 80.3% of
the calcium had reacted with 69.0% of the reaction occurring within
the first 5 minutes.
The above method was used to provide a product designated sample 3
(S# 3, Table 1), with a ratio of CaCO.sub.3 :Ca-montmorillonite of
1.8:1 using 1:1 CaO:Ca-montmorillonite using the above procedure.
The sample was tested for SO.sub.2 reactivity following the above
procedure. Under reaction conditions of example 1, sample 3 gave a
total CaCO.sub.3 conversion of 74.9% after 1 hour of reaction with
65.5% of the reaction occurring within the first 5 minutes.
The composites isolated in these methods are found by XRPD methods
to be rich in CaCO.sub.3. By allowing the mixtures to stir longer
time durations in water and also drying the slurries formed, in
air, initially formed Ca(OH).sub.2 had converted to much more
reactive CaCO.sub.3.
EXAMPLES 4 and 5
The preparation of Ca(OH).sub.2 /CaCO.sub.3 -Ca-montmorollonite
composites with varying base:clay ratios are described in these
examples.
The procedure is similar to Example 1. To a stirred 100 ml aqueous
clay slurry containing 1.8 g of Ca-montmorillonite (1.8% w/w clay
slurry), 5.4 g Mississippi lime was added. The mixture was stirred
at room temperature for further 3 hours and filtered through a fine
porosity filter paper under vacuum using a buchner funnel. The
filter cake was dried in an oven for 6 hours at 110.degree. C.
XRPD of the isolated product showed the composite consists of
mainly Ca(OH).sub.2 and small amounts of CaCO.sub.3.
The sample was tested for SO.sub.2 reactivity following the above
procedure. Under reaction conditions of example 1, sample 4 gave a
total conversion of 78.5% Ca sites to CaSO.sub.4 after 1 hour of
reaction with 61.0% of the reaction occurring within the first 5
minutes (S# 4, Table 1).
Using a similar procedure of above, the composite, 3:1
CaO:Ca-montmorillonte was prepared using a 5% w/w
Ca-montmorillonite slurry instead of 1.8% w/w. The product was
isolated as above and XRPD results showed that composite contained
predominantly Ca(OH).sub.2 and small amounts of CaCO.sub.3. The
sample was tested for SO.sub.2 reactivity following the above
procedure and gave a total CaO conversion of 75.6% after 1 hour of
reaction with 52.3% of the reaction occurring within the first 5
minutes (S# 5, Table 1).
The composite prepared using 1.8% w/w clay slurry showed enhanced
SO.sub.x reactivity than the composite prepared using 5% w/w clay
slurry. Much diluted clay slurries facilitate the proper dispersion
of base particles within the clay platelets and hence show higher
SO.sub.x reactivity.
EXAMPLE 6
The preparation of a Ca(OH).sub.2 /Ca-montmorillonite clay
composite using a Ca(OH).sub.2 as the base precursor is described
in this example.
Example 6 provided a product, designated sample 6, that was
prepared utilizing the procedures of Example 2. However,
Ca(OH).sub.2 from Columbus Chemical Industries, Inc., Columbus,
Wis. was used as the CaCO.sub.3 precursor instead of CaO. The ratio
of CaCO.sub.3 :Ca-montmorillonite was 4.0:1 after preparation.
Sample 4 was tested for SO.sub.2 uptake following the procedure
cited in Example 1. Sample 6 (S# 6, Table 1) exhibited a total
CaCO.sub.3 conversion of 75.9% after 1 hour of reaction with
SO.sub.2 with 61.9% occurring within the first 5 minutes.
The SO.sub.x reactivity of this compound is comparable to the
reactivities observed for the composites isolated according to
Examples 1 to 3, demonstrating that Ca(OH).sub.2 can be replaced
for CaO in preparing CaCO.sub.3 containing clay composites.
EXAMPLE 7
The preparation of CaCO.sub.3 /Na-montmorillonite composite is
described in this procedure.
Example 7 provided a product, designated sample 7, that was
prepared utilizing the procedures of Example 2, except that
Na-montmorillonite from Crook County, Wyo., USA was used in place
of Ca-montmorillonite. A ratio of 5.4:1 of CaCO.sub.3
:Na-montmorillonite was achieved by using 3:1
CaO:Na-montmorillonite. This sample 7 exhibited a total CaCO.sub.3
conversion of 78.6% after 1 hour of reaction with SO.sub.2 with
66.3% occurring with the first 5 minutes.
By comparing the SO.sub.x reactivity of this composite with the
composite prepared using Ca-montmorillonite with same base to clay
ratio (S# 1, Table 1), it is evident that composite containing
Na-montmorillonites show improved reactivities.
EXAMPLE 8
The preparation of Ca(OH).sub.2 /CaCO.sub.3 /Na-Montmorillonite is
described in this procedure.
Example 8 provided a product, designated sample 8 (S# 8, Table 1),
that was prepared utilizing the procedures of Example 4, except
that 1.8% w/w slurry of Na-montmorillonite from Crook County, Wyo.,
USA was used in place of Ca-montmorillonite. A ratio of 3:1 of
CaO:Na-montmorillonite was obtained by adding 5.4 g of Mississippi
lime, CaO to a stirred clay slurry containing 1.8 g of
Na-montmorillonite in 100 ml of water. The resultant slurry was
stirred three hours at ambient temperatures and filtered through a
fine porosity filter paper under vacuum using a Buchner funnel. The
filter cake was dried in an oven for 6 hours at 100.degree. C. The
product (S# 8, Table 1) exhibited a total CaO conversion of 72.2%
after 1 hour of reaction with SO.sub.2 with 53.7% occurring with
the first 5 minutes.
The Na-montmorillonite containing composite isolated in this manner
were found to be less brittle than the composites formed from
Ca-montmorillonite.
EXAMPLES 9 to 11
The preparation of CaCO.sub.2 /Ca-montmorillonite and CaCO.sub.3
/Na-montmorillonite by introducing carbon dioxide gas through
Ca(OH).sub.2 /Clay slurries are described in these examples.
To a stirred 100 ml aqueous clay slurry containing 1.8 g of
Ca-montmorillonite (1.8% w/w clay slurry), 5.4 g Mississippi lime
was added. The mixture was stirred at room temperature for further
3 hours while bubbling carbon dioxide gas through the solution.
XRPD studies of the samples isolated at different time intervals
from the above reaction mixture showed that conversion of
Ca(OH).sub.2 to CaCO.sub.3 had completed after about 30 minutes.
The precipitate was filtered through a fine porosity filter paper
under vacuum using a Buchner funnel. The filter cake was dried in
an oven for 6 hours at 100.degree. C.
XRPD of the isolated product showed the composite consists of
mainly CaCO.sub.3 and small amounts of Ca(OH).sub.2. This sample 9
exhibited a total CaO conversion of 84.2% after 1 hour of reaction
with SO.sub.2 with 61.5% occurring with the first 5 minutes.
Using a similar procedure 3:1 CaO/Ca montmorillonite composite was
prepared using 5% w/w Ca-montmontmorillonite instead of 1.8% clay
slurry. The product isolated (Sample 10, S# 10 in Table 1)
exhibited a total CaO conversion of 82.4% after 1 hour of reaction
with SO.sub.2 with 60.23% occurring with the first 5 minutes.
In another method 1.8% Na-montmorillonite from Crook County, Wyo.,
USA was used in place of 1.8% Ca-montmorillonite in the above
procedure. This sample 11, exhibited total CaO conversion of 78.6%
after 1 hour of reaction with SO.sub.2 according to the conditions
given in example 1 with 68.7% conversion occurring within first 5
minutes.
The composites isolated in this manner, by purging CO.sub.2 gas
through the Ca(OH).sub.2 /Clay slurries found to be rich in
CaCO.sub.3. Furthermore, they show improved SO.sub.x reactivities
than the composites isolated without purging CO.sub.2 gas (S# 4,
Table 1). Moreover, the composite prepared using 1.8% w/w clay
slurries showed enhanced SO.sub.x reactivities, compared to the
composites prepared using 5% w/w clay slurries, due to the improved
dispersions of base particles over clay supports.
EXAMPLE 12
The preparation of dry physical mixture of CaO/Ca-montmorillonite
is described in this example.
A dry physical mixture of CaO and Ca-montmorillonite in a 1:1
weight ratio was prepared by grinding. After preparation the
mixture was stored in a sealed vial to limit its exposure to air.
Prolonged exposure of CaO to air causes the formation of
Ca(OH).sub.2 or CaCO.sub.3 indicated by characteristic diffraction
lines in the X-ray powder pattern. The preparation was tested for
reactivity towards SO.sub.2 following the conditions cited in
Example 1. The sample designated, sample 12 (S# 12, Table 1), had a
total CaO conversion of 40.0% after 1 hour of reaction with
SO.sub.2 with 25.7% occurring during the first 5 minutes.
The SO.sub.x uptake observed for this dry physical mixture
containing 1:1 CaO/Ca-montmorillonite is much lower than that
observed for the composite prepared using same compositions (S# 3,
Table 1) suggesting that the physical mixtures of clay and base are
inferior to the clay/base composites.
EXAMPLE 13
The preparation of dry physical mixture of Ca(OH).sub.2
/Ca-montmorillonite is described in this example.
A dry physical mixture of Ca(OH).sub.2 from Columbus Chemical
Industries Inc., Columbus, Wis. and Ca-montmorillonite in a weight
ratio, 1:1, was prepared by grinding. After preparation the mixture
was stored in a sealed vial to limit its exposure to atmospheric
CO.sub.2. A sample of the preparation, designated sample 13 (S# 13,
Table 1), was tested for reactivity towards SO.sub.2 following the
conditions cited in Example 1. Sample 13 has a total Ca(OH).sub.2
conversion of 53.0% after 1 hour of reaction with SO.sub.2 with
32.8% occurring within the first 5 minutes.
As before, this physical mixture also showed low SO.sub.x
reactivity compared to clay/base composites.
EXAMPLE 14
In this example a dry physical mixture containing CaO and Clay
prepared according to example 12 is exposed to air and water for
longer duration of time to provide enough time to react with
CO.sub.2 from air to form CaCO.sub.3.
This sample (S# 14, Table 1) involved the preparation of a paste,
using the minimum amount of water to thoroughly wet the sample. The
sample was then allowed to air dry at ambient temperature for 16
hours to encourage CaCO.sub.3 formation by reaction with
atmospheric CO.sub.2. An improvement in reactivity towards SO.sub.2
was observed for Sample 14, a total conversion of 55.8% was
observed after 1 hour of exposure to SO.sub.2 with 39.3% occurring
within the first 5 minutes.
EXAMPLE 15
The effect of a CO.sub.2 atmosphere on a wetted mixture of
Ca(OH).sub.2 :Ca-montmorillonite, 4:1 for SO.sub.2 removal from
flue gas was tested in this example.
A dry physical mixture of Ca(OH).sub.2 :Ca-montmorillonite, 4:1,
was prepared by grinding CaO and Ca-montmorillonite in a mortar and
pestle, followed by exposure to moist air. This resulted in the
conversion of CaO to Ca(OH).sub.2 as discussed before. A paste was
prepared by wetting the mixture with a minimum amount of water to
thoroughly wet the sample. CO.sub.2 was then blended into the paste
for 16 hours at room temperature. The paste was then air dried at
approximately 50.degree. C. An XRPD pattern of the preparation
indicated a mixture of Ca(OH).sub.2 and CaCO.sub.3 was present with
Ca(OH).sub.2 being the dominant species. Decomposition of the two
species using thermal gravimetric analysis indicated that 85% of
the calcium was present initially as Ca(OH).sub.2. A total
conversion of 80.6% was observed after 1 hour of exposure to
SO.sub.2 with 50.7% occurring within the first 5 minutes (S# 15,
Table 1), following the procedure cited in Example 1.
EXAMPLE 16
A dry physical mixture of CaO:Ca-montmorillonite, 3:1, was prepared
by grinding CaO and Ca-montmorillonite in a mortar and pestle. A
wet paste was prepared from the mixture immediately following
grinding. CO.sub.2 was passed through the slurry for 20 hours at
80.degree. C. The paste was then air dried at approximately
50.degree. C. An XRPD pattern of the preparation indicated a
mixture of Ca(OH).sub.2 and CaCO.sub.3 was present with CaCO.sub.3
being the dominant species. A total conversion of 73.4% was
observed after 1 hour or exposure to SO.sub.2 with 53.4% occurring
within the first 5 minutes, following the procedure cited in
Example 1.
By comparing the SO.sub.x reactivity data for the composites
isolated in Examples 15 and 16, it is evident that improved
SO.sub.x reactivities are obtained when the dry physical mixtures
(from Examples 12 and 13) were treated with water to convert CaO to
Ca(OH).sub.2 and dispersed over clay.
TABLE 1
__________________________________________________________________________
Activity Base/Clay Composites For Removal of SO.sub.2 from a Gas
Stream.sup.a. Medium Base/Clay % Conversion.sup.b S. # Used
Precursor Base/Clay Wt. Ratio 5 min. 60 min.
__________________________________________________________________________
1 water CaO CaCO.sub.3 / 5.4:1 58.9 74.2 suspension (air) Ca-mont.
(0.25%).sup.e 2 water CaO CaCO.sub.3 / 8.9:1 69.0 80.3 suspension
(air) Ca-mont. (0.2%) 3 water CaO CaCO.sub.3 / 1.8:1 65.5 74.9
suspension (air) Ca-mont. (0.2%) 4 water CaO CaCO.sub.3
/Ca(OH).sub.2 3:1.sup.c 61.0 78.5 suspension (air) Ca-mont. (1.8%)
5 water CaO CaCO.sub.3 /Ca(OH).sub.2 3:1.sup.c 52.3 75.6 suspension
(air) Ca-mont. (5%) 6 water Ca(OH).sub.2 CaCO.sub.3 / 4.0:1 61.9
75.9 suspension (air) Ca-mont. (0.2%) 7 water CaO CaCO.sub.3 /
5.4:1 66.3 78.6 suspension (air) Na-mont. (0.2%) 8 water CaO
CaCO.sub.3 /Ca(OH).sub.2 3:1.sup.c 53.7 72.2 suspension (air)
Na-mont. (1.8%) 9 water CaO CaCO.sub.3 / 3:1.sup.c 61.5 84.2
suspension (CO.sub.2 Ca-mont. (1.8%) purge) 10 water CaO CaCO.sub.3
/ 3:1.sup.c 60.2 82.4 suspension (CO.sub.2 Ca-mont. (5%) purge) 11
water CaO CaCO.sub.3 / 3:1.sup.c 68.7 78.6 suspension (CO.sub.2
Na-mont. (1.8%) purge) 12 dry CaO CaO/Ca-mont. 1.0:1 25.7 40.0
blend 13 dry Ca(OH).sub.2 Ca(OH).sub.2 / 1.0:1 32.8 53.0 blend
mont. 14 paste CaO (Ca(OH).sub.2 / 1.0:1.sup.c 39.3 55.8 CaCO.sub.3
/Ca-mont. 15 paste Ca(OH).sub.2 (Ca(OH).sub.2 / 4.0:1.sup.c 50.7
80.6 CaCO.sub.3)/ Ca-mont. 16 paste CaO (Ca(OH).sub.2 / 3.0:1.sup.c
53.4 73.4 CaCO.sub.3)/ Ca-mont 17 CaO 14.6 43.7 18 CaCO.sub.3.sup.d
31.2 31.8
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.sup.a At 900.degree. C.; 5000 ppm SO.sub.2. .sup.b CaO + SO.sub.2
+ O.sub.2 .fwdarw. CaSO.sub.4. .sup.c Precursor/clay ratio. .sup.d
CaO + 2CH.sub.3 COOH .fwdarw. Ca(CH.sub.3 COO).sub.2 + H.sub.2 O
Ca(CH.sub.3 COO).sub.2 + Na.sub.2 CO.sub.3 .fwdarw. CaCO.sub.3 +
2NaOOCCH.sub.3. .sup.e Conc. of clay slurry % w/w.
It is intended that the foregoing description be only illustrative
and that the present invention be limited only by the hereinafter
appended claims.
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